don't update when not mounted

Author: willmcgugan

src/textual/app.py

@@ -762,8 +762,8 @@ def __init__(
         perform work after the app has resumed.
         """
 
-        self.set_class(self.dark, "-dark-mode", update=False)
-        self.set_class(not self.dark, "-light-mode", update=False)
+        self.set_class(self.dark, "-dark-mode")
+        self.set_class(not self.dark, "-light-mode")
 
         self.animation_level: AnimationLevel = constants.TEXTUAL_ANIMATIONS
         """Determines what type of animations the app will display.

src/textual/dom.py

@@ -1581,9 +1581,9 @@ def set_class(self, add: bool, *class_names: str, update: bool = True) -> Self:
             Self.
         """
         if add:
-            self.add_class(*class_names, update=update)
+            self.add_class(*class_names, update=update and self.is_attached)
         else:
-            self.remove_class(*class_names, update=update)
+            self.remove_class(*class_names, update=update and self.is_attached)
         return self
 
     def set_classes(self, classes: str | Iterable[str]) -> Self: